The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Velocity measurements using a Laser Doppler technique do not inherently resolve the direction of the velocity. The current methods to determine the sign of the velocity tend to be complex, bulky and/or require significant electrical power. Additionally, previously developed systems and methods usually impose a secondary peak at zero velocity. This tends to create a dead band in velocity around zero velocity. For applications where the velocities are low, the dead band in velocity around zero velocity is a significant performance limitation of such systems.
There are generally two classes of direction resolution solutions. The first class measures both the signal and quadrature of the electrical beat frequency. A Fourier transform of a time series including both signal and quadrature is generated that has both positive and negative frequencies. The second class of solutions imposes a fixed, offset frequency on a local oscillator of the system. The difference between a detected beat frequency and the fixed offset frequency can be both positive and negative, thus resolving the direction ambiguity. Two common methods are used for imposing a frequency offset. An Acousto-Optic Modulator can be used to impose an offset frequency in the range of about 30-250 MHz. A rotating radial grating can be used to impose lower frequency offsets.
The first class of solutions described above (i.e., measuring both signal and quadrature) also requires a complex optical subsystem which cannot be implemented in an optical fiber. With the second solution mentioned above, an acoustic optic-modulator is required. An acoustic optic-modulator is a relatively large and heavy component and typically requires several watts of radio frequency (RF) power to operate. It is also somewhat electrically noisy. The use of a rotating radial grating also has limitations associated with it. For one, this component requires a precision motor to rotate the grating at a fixed frequency. Without extreme care, the rotating grating is susceptible to vibration. Moreover, the use of a rotating radial grating is not easily adapted to an optical fiber system.